The invention pertains generally to the field of printing presses and, more particularly, to ink fountains for printing presses.
In rotary offset printing presses, a thin film of ink is continuously applied to a printing plate on which has been formed an ink receptive image. The thin film of ink tends to adhere only to the image portion of the plate. The plate is carried on a rotating cylinder or drum. The printing plate rolls the image directly on a printing substrate (e.g. paper or mylar) or on an impression blanket cylinder, which in turn rolls it onto paper. Paper is fed along a transport in either discrete sheets or a continuous web.
The ink film applied to the printing plate must be of uniform thickness and continuous for printing an image of consistent quality on the paper. To create and deliver this thin film of ink, a train of rollers takes ink from a reservoir called an ink fountain and, during transport of ink from one roller to the next, smooths it into the continuous and uniform thin film. The ink is metered from the fountain to the ink train at a rate which is sufficient to maintain uniformity and continuity in the film. Most ink is metered from fountains using a similar method. An end of a substantially flat plate, called a blade, is placed under and to one side of a rotating roller, called a fountain roller. The blade is angled upwardly to trap the ink against the fountain roller. Cheeks on opposite sides of the blade and roller create, with the blade and the fountain roller, the ink reservoir. The blade is positioned so that, as it engages the fountain roller, a narrow gap is formed between it and the roller. The fountain roller rotates toward the blade, taking with it a film of ink adhering to its surface. The size of the gap between the roller and the blade determines the amount or thickness of the film which is carried by the fountain roller and delivered to the ink train via, typically, a ducting roller. The position of the edge of the blade with respect to the fountain roller is adjustable to change the metering rate. As consumption rates usually varies across the fountain due variations in the image being printed, the metering edge of most blades is flexible so that ink can be metered at different rates along its width. A row of screws or adjustable pins, called keys, are used to slightly bend or pressure the edge at discrete locations and thereby contour the edge of the blade and vary the gap or pressure between the blade and fountain roller. Each key can be used to adjust metering along a predefined interval or segment of the blade.
Printing ink is a oily, viscous substance. It is tacky so that it will properly adhere to the image areas on the printing plate. For example, ink used to print newspapers is the least viscous, usually in the range of 50 to 80 poise. Ink for letter presses and heat-set inks employed for web offset printing have viscosities in the range of about 150 to 200 poise. Inks for sheet fed, lithographic offset printing presses are the most viscous, usually in the range of 250 to 300 poise. Newer xe2x80x9cwaterlessxe2x80x9d inks, which eliminate the need for conventional dampening systems to apply a thin film of water to the non-image areas of the printing plate, are highly viscous, gel-like substances which do not flow. Due to printing inks viscous nature and tendency to stick to surfaces in the ink fountain, the ink will tend not to flow easily to low spots, especially when the level of the ink in the fountain is low or the printing ink is of the very viscous type used in sheet fed, lithographic offset presses. The ink level in the fountain can develop low spots, especially as the overall level of ink in the fountain drops. A low spot will lead to a tinning of the supply of film to the ink train, which in turn may result in a film which is not uniform or is discontinuous being delivered to the printing plate, resulting in poor quality prints.
In smaller and mid-size offset presses, especially sheet fed offset presses, a pressman manually scoops ink out of a can and spreads it along the width of the ink fountain in a thick layer at the commencement of a run. Pressmen will naturally tend to put more than enough ink in the fountain for the job to guard against development of low spots which could result in wasted prints of inferior quality. Consequently, it is not unusual for a substantial amount of ink leftover in the ink fountain at the end of a run. This ink is almost always discarded. It may be specially mixed for the particular job and it tends to quickly oxidize. A portion of ink in the fountain is exposed to air and will have already begun to oxidize, even if agitated or stirred in the ink fountain to reduce the effects of oxidation.
By some estimates, as much as seventy percent (70%) of ink used in printing is discarded. Discarded ink imposes a substantial cost on printing in two ways. First, printing ink is expensive and constitutes a large portion of the total cost of a printing job. Second, printing ink is a hazardous substance and is environmentally harmful. Disposal of discarded ink in an environmentally sensitive way is expensive and, in many places, mandated by government regulations.
Automatic systems have been used for replenishing ink in ink fountains on large printing presses, especially newspaper and other large web printing presses which consume large quantities of lower viscosity ink. These systems operate to maintain a predetermined quantity of ink in the system by measuring the level of the ink in the ink fountain and opening a valve to pump ink into the fountain from an external drum or supply when the ink drops below a preset level. Several techniques have been used in such apparatus to sense the level of ink, including floats, tactile or mechanical sensors, pneumatic sensors, capacitive sensors and ultrasonic sensors. Generally, sensors which require physical contact with the ink have been unreliable due, at least in part, to the viscosity of the ink. The invasiveness of such sensors may also interfere with the metering function of the fountain. Ultrasonic transducers which determine distance using conventional ranging methods are beset by a number of problems commonly associated with acoustic ranging equipment. Acoustic signals are sensitive to air disturbances which may deflect or reflect the signal. They are also sensitive to ambient temperature fluctuations which alter the velocity of the acoustic waves. Air disturbances and temperature fluctuations may be caused, for example, the heat given off by the printing press and other environmental influences. Disturbances in the surface of the ink caused by, among other things, mechanical agitators used to stir the ink also cause inaccurate readings. Acoustic signals will also tend to resonate or ring if the distance between the sensor and the surface of the ink is small, making timing of the return signal difficult and unreliable.
The objective of such systems is not to avoid discarding ink, however. In large runs, the amount left over in the ink fountain is not likely to be a large percentage of the amount of ink dispensed from a bulk supply. Rather, it is supplying large quantities of ink to reservoirs of limited capacity for large printing runs. Such automatic system will tend to maintain a maximum amount of ink in an ink fountain in order to avoid any risk of ink starvation. Such apparatus do not address the special problems of maintaining only a minimum level of ink in the ink fountains, especially when such ink is highly viscous.
The invention provides for an apparatus and method for automatically maintaining a minimum level of ink in an ink fountain and thereby avoid wastage, especially when using ink which is viscous or does not flow well. The preferred embodiment of the invention has a number of different aspects, which, singly or in combination with one or more of the other aspects, give it advantages over the prior art, especially when used on printing presses running smaller jobs and/or use using particularly viscous ink. Several of these aspects and their advantages are summarized below.
According to one aspect, an ink fountain level sensor is mounted for lateral movement across the ink fountain. It moves across the ink fountain, measuring the level of ink along the width of the fountain. When a low ink level is detected, an ink dispenser deposits additional ink into the fountain. A lower level of ink within the fountain can be set, especially when using highly viscous ink, as the sensor will be able to guard against low spots developing which would result in ink starvation.
According to another aspect, an ink dispenser is mounted for lateral movement across the ink fountain. Ink may thereby be delivered immediately and directly to low spots, if and when they develop. It effectively is delivered directly to the sections of the ink fountain consuming most of the ink. As the ink need not flow from a fixed dispense location, a lower level of ink can be maintained in the fountain and consumption demands for different portions of the fountain met. In combination with an ink fountain level sensor scanning the ink level, the dispenser may be directed to the low spot. When mounted for movement with the ink fountain level sensor, the dispenser may remedy the low level soon after detection.
Furthermore, and according to another aspect, an ink dispenser deposits ink on a fountain roller. The roller carries the ink toward, and forces it into, the narrow convergence between the fountain roller and the blade in the ink fountain. Thus, a large head of ink need not be maintained to push it toward the metering gap between the blade and fountain roller of a conventional ink fountain. Indeed, a small bead of ink may be maintained in the gap when the ink dispenser traverses the fountain and deposits small amounts of ink as needed to maintain the bead.
According to another aspect, ink level in an ink fountain is sensed using a photoelectric proximity sensor which reflects an optical beam off of surfaces. The beam may be aimed such that it determines whether there is ink between it and a certain predetermined distance and determines whether the ink level is low based on where the reflected beam hits an optical detector. The beam can be focused or aimed at a small areas. It tends not to be subject to ambient disturbances which affect ultrasonic waves. The method of measuring offers a high resolution and accuracy. When traversed across the ink fountain, it is well suited for detecting low spots in the ink, especially when the ink is maintained as a bead of narrow cross-section in the convergence between a fountain roller and a blade. It also has advantages over the prior art. For example, ultrasonic waves used in ultrasonic sensors tend to spread. Thus, they tend not to have sufficient resolution to discriminate between the ink fountain and a minimum ink level in the ink fountain, especially a small bead nestled between a blade and fountain roller. It is also difficult to use capacitive or inductive sensors in such situations since they will tend to give erroneous readings when positioned too close to metal in the ink fountain.
Finally, irregularities in the surface of ink in an ink fountain cause unpredictable deflections in an optical beam transmitted by an photoelectric proximity sensor traversing the length of the fountain. Such deflections result in false readings: sometimes the beams reflection is such that it appears that the ink level is closer than it actually is; sometimes the reflection indicates that the ink level is farther than it actually is. To better assure that the level of ink is maintained at a preset level, the ink fountain level sensor is, according to another aspect of an embodiment of the invention, sampled multiple times over a predefined segment or interval. Ink is dispensed when a predefined percentage of samples taken within the segment indicates a low ink level; or, conversely, ink is not dispensed when a predefined percentage of samples indicates that the ink is above a preset level. Although the samples can be taken over a series of fixed, end-to-end segments, the calculation is preferably done on a segment moving with the ink fountain level sensor. In effect, it is a moving window of the last number of samples constantly moving, in effect, a single segment. A running percentage is calculated by taking the value of next sample and dropping the value of last sample, and determining the percentage of samples indicating that either the ink in the fountain is low or high. This moving window avoids the possibility of a low spot developing at a boundary between otherwise fixed segments.
The forgoing summary is intended only to aid in the understanding of advantages of various aspects of the preferred embodiments exemplifying the invention and not to limit the scope of the invention as set forth by the appended claims. The invention, as claimed, may have other or additional advantages which will be apparent from the following description of the preferred embodiment made in reference to the accompanying drawings, in which: